In general, commercially pure titanium is prepared usually in the form of a large cast product by using titanium sponge obtained by a Kroll process and titanium scraps as melting raw materials and melting them by vacuum arc remelting (VAR) and electron beam remelting (EBR). In this connection, the form of the cast product is limited to a cylindrical cast product in the case of VAR. On the other hand, the materials can be casted into a rectangular cross-section cast product, that is, a slab in the case of EBR.
Further, when such the large cast product as described above is used as a raw material to manufacture titanium materials such as titanium sheets and the like, the large cast product is subjected, if necessary, to cutting treatment of a surface and then to slab rolling or forging at a hot temperature to deform the large ingot into a slab having a form and a size which are suited to subsequent hot rolling. A hot working process carried out by the above slab rolling or forging is referred to as a breakdown process in this application. Further, usually, the slab is subjected to cutting treatment for removing a surface thereof by about several mm by cutting work in order to remove an oxide layer and an oxygen-enriched layer which are formed on the surface of the slab after the breakdown process, and then the slab is subjected to hot rolling.
However, the above conventional method requires a great deal of time and costs for the breakdown process carried out by slab rolling or forging for deforming the large cast product into a form and a size which are suited to hot rolling, and this has largely hindered an improvement in a productivity and a reduction in a cost in manufacturing titanium sheets.
On the other hand, in recent years, a technique for manufacturing a relatively thin slab-shaped cast product, that is, a titanium cast product having a form and a size which make it possible to subject the cast product to hot rolling as it is, by a DC slab casting method (direct casting method) is being established as a method for casting a slab-shaped cast product instead of casting such the large ingot as described above. According to the DC slab casting method, molten titanium obtained by melting titanium in a hearth by an electron beam and the like is continuously injected into a water-cooled copper mold maintained to be a vacuum atmosphere, and a part of the molten titanium solidified in the water-cooled copper mold is continuously pulled out from a lower end side of the mold to obtain a slab-shaped cast product having a prescribed length.
Applying the DC slab casting method carried out by the above EBR and the like under vacuum makes it possible to omit the breakdown process which has conventionally been required, which results in making it possible to improve a productivity in manufacturing a titanium sheet and reduce a manufacturing cost thereof.
Further, there is the problem that even when a slab (omitting the breakdown process) obtained by applying the DC slab casting method carried out by the EBR and the like under vacuum as described above is subjected to hot rolling, the surface property of a hot rolled sheet after hot rolling is not necessarily improved. That is, there is the problem that many small and large overlapping flaws having a length of several mm to about 10 mm are formed on the surface of the hot rolled sheet. Such many overlapping flaws formed on the surface shall be referred to as surface flaws in this application. Such the surface flaws formed on the hot rolled sheet are considered to originate in coarse cast microstructure of a cast slab. That is, a slab manufactured without passing through the breakdown process in which hot working is carried out has cast microstructure composed of coarse crystal grains as cast, and even if the surface thereof is subjected to cutting work to make undulations on the surface smaller, the coarse microstructure is present in the surface layer after cutting. It is considered that the surface flaws are formed on the hot rolled sheet due to the cast micro structure of such the coarse cast microstructure in the surface layer.
In this connection, a specific factor in which surface flaws are formed on a hot rolled sheet due to coarse cast microstructure is considered to be attributable to that relatively large dents are formed in a boundary part between a mother phase and a twin crystal because of a large misorientation between the mother phase and the twin crystal and a coarse hot twin crystal formed in the beginning of hot rolling and metal is overlapped on the above dents to turn into surface flaws as subsequent hot rolling proceeds.
On the other hand, there has already been proposed some methods in which a surface layer of a titanium slab for hot rolling which is obtained without passing through the breakdown process is subjected to reforming treatment before hot rolling in order to prevent surface flaws from being formed on a surface of a hot rolled sheet after hot rolling.
For example, in Patent Literature 1, it is proposed that a surface of a titanium slab for hot rolling is struck (subjected to plastic deformation) at a room temperature by a steel tool with a tip curvature radius of 3 to 30 mm or a steel ball having a radius of 3 to 30 mm, which provides the slab with dimples having an average height of 0.2 to 1.5 mm and an average length of 3 to 15 mm in a contour curve element of a undulation. In the method proposed above, the surface layer of the titanium slab is provided with prescribed plastic strain at the room temperature by the steel-made tool or the steel ball each described above to thereby recrystallize the surface layer in subsequent heating prior to hot rolling and form fine microstructure, whereby dents can be prevented from being formed due to such the coarse microstructure as described above. Accordingly, even when the breakdown process is omitted, surface flaws of a hot rolled sheet can be reduced.
In Patent Literature 2, there is proposed a method in which a surface of a titanium slab for hot rolling, especially a surface of a side which is a surface to be rolled in hot rolling is provided with high energy by high frequency induction heating, arc heating, plasma heating, electron beam heating, laser heating and the like to melt only the surface layer by a depth of 1 mm or more and in which the surface is immediately quenched and solidified again. In the case of the method proposed above, titanium has naturally a melting point which is higher a β transformation point, and therefore as the surface is molten, a heat affected zone (HAZ) layer of a lower side (parent metal side) than the molten layer on the surface is heated as well to the β transformation point or higher and subjected to β transformation. In the method proposed above, the surface layer of the titanium slab for hot rolling is molten, whereby the surface is smoothed; further, the molten layer is then quenched by removing heat to the parent metal side and solidified; and at the same time, the HAZ layer (β phase) at a lower side is quenched, whereby the molten layer and the HAZ layer are turned into fine transformation microstructure (usually fine acicular microstructure). Then, the surface layer which has been refined in the manner described above is recrystallized in the subsequent reheating prior to hot rolling and turned into granular microstructure (equiaxed grain microstructure) having a fine and random orientation. Accordingly, dents attributable to the coarse microstructure can be prevented from being formed, and the surface flaws on the hot rolled sheet after hot rolling can be overcome as well.